An implantable cardiac device is a medical device that is implanted in a patient to monitor electrical activity of the heart and to deliver appropriate electrical and/or drug therapy, as required. Implantable cardiac devices include, for example, pacemakers, cardioverters and defibrillators. The term “implantable cardioverter defibrillator” or simply “ICD” is used herein to refer to any implantable cardiac device.
An ICD employs a battery to power its internal circuitry and to generate electrical therapy. The electrical therapy can include, for example, pacing pulses, cardioverting pulses and/or defibrillator pulses.
When a battery is manufactured, its energy capacity is known. Specifically, it is known how many Ampere-Hours of energy the battery can deliver. Based on the known battery energy capacity and based on predicted usage, battery life can be predicted and a replacement interval established. With this approach, a conservative margin in remaining battery life is observed to prevent device failure due to a depleted battery. Alternatively, actual battery usage can be tracked, and the device can be replaced when the actual remaining energy capacity of the battery falls below a predetermined threshold. This second approach of tracking battery usage and replacing a device when replacement is actually required is preferred since it reduces unnecessary device replacements. It could, also, be used to inform the physician of unexpected battery depletion or excessive current drain that might be a sign of malfunction.
Conventional methods for tracking battery usage use estimation techniques to determine how much energy is left in the battery. As previously mentioned, the estimation techniques are not accurate and require observation of conservative margins. Other conventional systems measure battery voltage and use the voltage measurement as an indicator of how much energy is left in the battery. This method is problematic because of unexpected drops and hikes in voltage within the battery circuit which can lead to inaccurate battery life predictions.
What is needed is a better and more accurate system and method for keeping track of the battery usage, so that the end-of-life of the battery can be predicted with greater precision.